Gingerich's team concludes that the tiny primates first appeared in China at the beginning of an ancient warming period that began 55 million years ago.

The animals appeared in Europe slightly later and then made their way to North America 20,000 years into the warming event.

"So you have a suggestion that it's appearing earlier in China than in Europe, and earlier in Europe than in North America," Gingerich said.

In addition, he says, the Chinese animals appear to be more primitive than the European ones, which in turn appear more primitive than North American specimens.

This indicates that Teilhardina migrated west from China to Europe and then to North America, covering 12,000 miles (20,000 kilometers) in 20,000 years.

That, Gingerich said, represents "very rapid dispersal," occurring at the same pace at which animals move into new territories today.

"It shows that the process of dispersal was similar then to what it is now," he said.

Furthermore, it shows that the animals spread during the warm period, when elevated temperatures would have created land bridges in parts of today's Arctic.

Ancient Global Warming

The ancient global warming period, known to scientists as the Paleocene/Eocene Thermal Maximum, lasted approximately 100,000 years.

During the first 20,000 years, the Earth warmed by about 9 degrees Fahrenheit (5 degrees Celsius) before returning to previous levels.

Many other mammals, including ancestral horses, spread across Asia, Europe, and North America at about the same time, Gingerich points out.

"This is part of a bigger pattern," Gingerich said.

"The main connections between the Northern continents are at high latitudes. I would say that the [warming period] opened routes for dispersal. That's the simple explanation for the rapid dispersal."

The period of global warming can be traced in rocks from era by a telltale change in carbon isotopes caused by the rise in temperature.

This signature allows rocks from various parts of the global warming period to be matched up, even if they are from different parts of the planet.

"We can connect the Teilhardina fossils to their place and date them relative to each other with a precision I never thought would be possible this far back in the past," Gingerich said.

Not Enough Fossils?

Not everyone is convinced that the new study has accurately mapped the primates' journey.

Scott Wing, a paleobotanist at the Smithsonian Institution in Washington, D.C., thinks that the number of fossil primate specimens from that era is too small to let scientists pinpoint the animals' earliest appearance in any given location.

"That part seems pretty speculative to me," he said in an email.

But he says the new study does demonstrate that the primatesand presumably other mammalsdispersed during the warm interval.

The migration was enabled by the growth of warmer-climate vegetation on the far northern land bridges, he adds.

"For me, the take-home point is that mammals moved very rapidly around the world in association with global climate change," Wing said.